Abstract

Thorium,
cerium,
lanthanum and other rare earths have been extracted from monazite sands for a long time due to several reasons. At the end of the XIX century monazite sands from Brazilian beaches were concentrated gravimetrically to be clandestinely transported to Europe to be used in the manufacture of fabric bags to be burned. Those bags when burning had their brightness enhanced by a mixture of thorium,
lanthanum and cerium. The thorium oxide
percentages found in monazite concentrates from several parts of the world range from 1 to 11%. While not burned or processed chemically monazite concentrate can be considered hazardous only due to gamma radiation and
(thoron) inhalation by those who stay very close to storage piles. In old monazite plants the thoron concentration in air reached levels higher than
Fortunately, the industrial hygiene improved with time and today's monazite separation plants do not present such high thoron concentrations. Old and recent data from rare earth processing plants suggest that occupational annual doses indoors may be up to two orders of magnitude higher than the worldwide average effective dose reported by the United Nations Scientific Committee on the Effects of the Atomic Radiation (UNSCEAR) due to the inhalation of thoron. Consumer products, like fabric bags used in gas or kerosene lamps have
concentrations of the order of
Remnants of the monazite industrial cycle can present
activity concentrations as high as
Piles of monazite concentrates were kept for strategic reasons during cold war times in several places of the territories of the United States and the former Soviet Union. Cleanup at those sites may be hazardous mostly due to high concentrations of thoron daughters that may be inhaled by workers. The paper will take a critical look at the naturally occurring radioactive materials (NORM) extant in the monazite cycle and its remnants.